Many wood species that are untreated are very susceptible to influences caused by the external environment. Untreated wood that is exposed to moisture and/or soil for sustainable periods of time will become weakened by attacks by various types of microorganisms or insects. It is therefore of importance to treat the less durable wood in order to increase its resistance against moisture and fungal attack. In addition wood which is exposed to Ultra Violet radiation is susceptible to discoloration and deterioration.
There exist a number of different treatment methods which will increase the resistance of wood. Chemical treatments of wood in order to increase the biological durability and strength have been used for a long time. Many different chemicals may be added. These chemicals are normally called fungicides and they will provide long-term resistance to organisms that cause deterioration of the wood. If it is applied correctly, it can extend the productive life of timber by five to ten times.
Another known method to improve the resistance of wood is to treat the wood at high temperatures to thermally modify the wood. During heat modification, certain organic compounds found in the wood will be removed and thereby decreases the possibility for fungi and rot to thrive on the wood. Thus, by heating wood to a certain temperature, it may be possible to make the ligno-cellulosic fibers less appetizing to fungi and insects. Heat modification can also improve the properties of the wood with respect to moisture both liquid and humidity, i.e. lower equilibrium moisture content, less moisture deformation, and improved weather resistance. One potential downside of thermally modified wood is the reduction in strength both in bending strength and surface hardness as a result of the modification process which might reduce the useful service life of the material.
Thus, there is also a need to further protect thermally modified wood against environmental influences. Today the most common way to protect solid wood products is by applying a polymer coating to the surface of the wood product. The most commonly used polymer coating is conventional paints and coatings available from both hardware stores and industrial producers. These coatings can be applied by hand or in a factory environment and are normally based on first applying a base (primer) coating and then one or two additional over coat layers to build a protective film on the surface of the substrate. This kind of coating is susceptible to the changing climatic conditions and over time erodes, cracks or peels from the substrate, leading to a need for periodic remedial maintenance. Another known method is to apply a layer of polyvinyl chloride (PVC) polymer on or around a solid wood product which is described in WO2007057029A1. A problem when coating solid wood, either if it is thermally modified wood or normal solid wood (kiln dried wood), with a polymer is dimensional change and interfacing strength between the substrate and polymer, i.e. the dimension of the solid wood and/or the coated polymer layer changes during time at differing rates which causes stresses at the point of interface which can lead to eventual deterioration in performance unless the wood substrate is fully encapsulated.
For standard wood fiber polymer composites (WPC's), a common problem is creep due to both the high weight and thermoplastic nature of the material based on fiber, thermoplastic resins and additives. Creep can be both weight, heat and moisture induced and often limits the use of standard wood fiber polymer composites from being used in more detailed and challenging applications where longer spans are needed and the products are required to carry their own weight such as in a cladding or façade board.
There is thus a need for an improved composite product comprising solid wood and a fiber polymer composite coating.